Many different devices have been developed for the purpose of measuring the viscosity of fluids. Many of these devices have vibrated a test rod or similar device in the fluid, rotated a test probe in a fluid, or used similar techniques. Typical of these prior art techniques are those described in U.S. Pat. Nos. 3,382,706; 3,762,429; 3,712,117 and 3,710,614. A major disadvantage of these techniques is that the test probe itself is affected by the density of the fluid under test as well as by the viscosity. This tends to cause some error in the measurement. An additional problem that occurs in devices of this type is that, although the basic definition of viscosity requires that the shear between two parallel plates, caused by the fluid, be measured, there is no readily available second plate with which to reference the shear waves. These factors tend to reduce the accuracy and reliability of measurements of this type.
Another viscometer known in the prior art is that described in U.S. Pat. No. 3,349,604. In this viscometer, changes in the resonant frequency of a vibrating probe disposed in the fluid are measured to provide an indication of the viscosity of the fluid itself. A disadvantage of this system is, as noted above, that the vibrational frequency is effected not only by the viscosity but also by the density of the fluid. This tends to introduce unwanted errors is measurement.
Another known type of viscometer or rheogoneometer uses rotating or oscillatory plates with the fluid disposed between the plates. The energy transferred from the first to the second plate is measured to provide an indication of the viscosity of the fluid disposed between the plates. While this is an apparatus which has many advantages over those described above, it nevertheless suffers from the disadvantage that it is not entirely accurate for the reason that it does not directly measure the shear, but rather measures a factor, energy transfer, which is the result of shear.
Accordingly, it is an object of this invention to obviate many disadvantages of the prior art viscometers.
Another object of this invention is to provide an improved apparatus for measuring the viscosity of fluids.